alternator issue

here's the issue I have been dealing with, I installed a new alternator on my small block 340. The Alternator is a single V pulley as well as the harmonic bal. and W/P. Now, with that being said the alternator sets back about 1/4 inch so the belt don't ride properly in the pulley. If I were using a double pulley it would line up perfect, and I know I have the right brackets...not real sure what the deal is. do I need a spacer as well... Any help would be great.